A high water treatment method has been established against Glugea plecoglossi-infected ayu. Plecoglossus altivelis, using artificially infected fish through oral administration of spores.Glugea cysts completely disappeared after fish were maintained at 29ﾟC for 5 days 11-26 days after the spore administration, followed by the same treatment 7 days after the first treatment. A comparable result was obtained when naturally infected fish were similarly treated.Ayu immuno-globlin was purified and an ELISA system has been established to detect antibodies against whole spores. The antibody was detected as early as 10 days after oral administration of spores to ayu. Since spores taken from the intestinal surface of ayu were confirmed by the Uvitex 2B staining, it was thought that the antibody was produced against the injested spores.Using ayu with a 30% prevalence of infection at the start, challenge tests were conducted through oral administration of live spores to ayu after treatment with high temperature water. As a result, no firm evidence was obtained that the fish had acquired immunity against the microsporidian infection through the treatment, though elevation of the antibody levels were recognized in some fish. The fact that control fish (treated but not challenged) had a significantly lower prevalence of infection and smaller number of cysts than challeged fish confirmed the efficacy of the treatment method.